1. Field of the Invention
This invention relates to a helical resonator filter accommodated within a metal casing, and more particularly it pertains to such a filter which is arranged to provide a desired characteristic without providing any partition wall formed with a window inside the casing thereof.
2. Description of the Prior Art
In prior art, it has been the practice that use is made of a metal casing provided with inside partition walls, each of which is formed with a window, to define compartments in which helical coils each wound on a hollow bobbin are disposed in such a manner as to be electromagnetically coupled to each other through the aforementioned windows, and that the size of each of the windows is so selected as to make the coupling coefficients suitable for achieving a desired characteristic. With such prior arrangement, however, cumbersome working operation is involved in providing partition walls such as mentioned above in the casing. Furthermore, the sizes of the windows must be changed to achieve different characteristics. Thus, the manufacturing cost of the filter is inevitably increased.